Balloon antenna

ABSTRACT

A phased array balloon antenna having an inner membrane coupled to an outer membrane and a phased array antenna connected to an inner membrane. The phased array antenna transmits an energy towards a reflective film on the outer membrane, reflecting the energy outwards and illuminating an area smaller than that illuminated by the phased array alone.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein may be manufactured and used by or forthe government of the United States of America for government purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

Large aperture antennas are needed in satellite based radars to focus aradar beam, due to the limitations on power available on the satellite.In order to place a large aperture antenna in space on a satellite theradar assembly has to be packaged to fit into the launch rocket as wellas be assembled or unfolded in space. This is very difficult andprohibitive for large aperture phased arrays.

Phased array radar systems are preferred in space based applications dueto the ability to electronically steer the array, thereby not requiringa movement of mass. Making a large phased array antenna that can beassembled or unfolded in space is very difficult task. There is a needto provide the effect and coverage of a large aperture phased arrayantenna while solving the problems of difficulty of installation inspace and limited power supply.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a phased array balloon antenna for space basedoperation according to an embodiment of the invention.

FIG. 2 is a diagram of G_(a) _(—) _(dB)(d_(spot))vs, d_(max)/mi.

FIG. 3 is a diagram of a phased array aperture.

DESCRIPTION OF THE EMBODIMENTS

Before explaining the disclosed embodiments of the present invention indetail it is to be understood that the invention is not limited in itsapplication to the details of the particular arrangement shown since theinvention is capable of other embodiments. Also, the terminology usedherein is for the purpose of description and not of limitation. In thefigures, the same reference numbers are used to identify the samecomponents.

Embodiments of the invention include a phased array balloon antenna anda method for using the antenna. A phased array antenna system is placedon an inner membrane of a balloon. The outer membrane of the balloon hasa reflective film suspended across a portion of it's volume or is linedwith a reflective film. The phased array operates as a large apertureantenna by reflecting the radar energy or beam from the phased arrayantenna off the reflective film so that it has the same effect as iflarge aperture antenna was in use. This approach permits a very smallphased array to operate as a large aperture phased array while utilizingloss power. A conventional phased array antenna coupled to a largereflector will operate for beaming purposes as a large phased arrayantenna. The balloon system shall be packed deflated and launched with asatellite and shall be deployed by inflating on arrival.

FIG. 1 illustrates a space based embodiment of the invention. A balloonantenna 100 is constructed of an inner membrane 120 and an outermembrane. 110. The outer membrane 110 is constructed so as to beinflatable and to hold a volume of gas. The inner membrane 120 supportsa phased array antenna 130. The outer membrane 110 has a reflector film150 suspended across a portion of the outer membrane's volume, having acurved shape determined by an inflation pressure differential betweenthe large compartment 140 and the small compartment 144 on the oppositesides of the reflective film 150. The reflective film is constructed ofa non-gas permeable material. A phased array antenna (represented by130) is connected and mounted on the inner membrane 120. The phasedarray 130 is directed outwardly from the center of the balloon towardsthe inside curved surface of the reflective film 150. The reflectivefilm 150 operates as a large aperture antenna and as the reflector forthe phased array radar 130 system connected to the inner membrane 120.

The phased array radar system antenna 130 transmits an energy towardsthe reflective film 150, reflecting the energy from the reflective film150 outwards and illuminating a target area smaller than the area (withmore energy per unit area) that would be illuminated by the phased arrayantenna operating alone. The phased array 130 shall transmit radarenergy towards the reflective film 150 on the outer membrane 110,reflecting the radar signal 180 onto the target area 160. The directionof the radar signal 180 is electrically steered by the phased array 130,which requires no moving parts, only a change in phase between theelements. It is noteworthy that since no mass is being relocated thephase array 130 is suitable for use on a satellite. The amplitude andphase of the radar energy is provided by the phased array 130. Theeffective curvature or shape can be adjusted smaller or larger as thecircumstances required by adjusting the pressure differential betweenthe large compact 140 and the small compartment 144. By adjusting theshape of the reflective film (thereby focusing the antenna), the limitedpower available on a space based radar can be concentrated in a desiredarea. Therefore, the phased array signal will have the effect on thetarget as if it were coming from a large aperture antenna. Otherembodiments may be constructed utilizing additional reflective films andadditional phased arrays within the balloon. In this way embodiments ofthe invention may be used for tracking and illumination of additionaltarget areas.

Another embodiment of the invention includes a method for illuminatingan area with radar energy including: providing a balloon antennacomprising at least one inner and an outer membrane, at least one phasedarray antenna connected each inner membrane, the outer membrane havingat least one reflective film and being inflatable; transmitting a radarenergy from each phased array antenna towards each reflective film;reflecting the radar energy outwards from the reflective film andilluminating a target area that is smaller than an area illuminated bythe phased array antenna; and changing the inflation of at least onecompartment within the outer membrane which changes the shape of thereflective films, thereby adjusting the target area illuminated by thephased array balloon antenna.

RESULTS OF SIMULATION

A calculated simulation was performed to evaluate the performance of anembodiment of the invention. The simulation assumed a radar illuminatorin a geosynchronous orbit, having no detection requirements, as could beused as one part of a bistatic system. Based on an illuminated targetarea of 25 miles in diameter, the member of elements and array size canbe found.

Constant and Units

$\begin{matrix}{c:={{3 \cdot 10^{8} \cdot \frac{m}{\sec}}{Speed}\mspace{14mu}{of}\mspace{14mu}{Light}}} & \; & {{{nmi}:}\; = {1852\mspace{20mu} m}} \\{{f:={10 \cdot {GH}}};\mspace{56mu}{\lambda:=\frac{c}{f}}} & \; & {\lambda = {0.03\mspace{20mu} m}}\end{matrix}$

Gain vs. Spot Size

One of the major concerns when operating from geosynchronous orbit iscontrolling the illuminated area (spot) size on the ground, whichdetermines the energy per unit area. As power in space is limited it isnecessary to limit the illuminated area to maintain detectability.

$\begin{matrix}{{d_{spot}:={1 \cdot {mi}}},\;{1.1 \cdot {{mi}.\; 25} \cdot m}} & {{d_{g}:=1},\;{{1.1..}\; 25}} & {H_{a}:=22800}\end{matrix}$ H_(alt) := 22800  m $\begin{matrix}{{{\theta_{3{dB}}\left( d_{spot} \right)}:\frac{d_{spot}}{H_{alt}}}\;} & {\mspace{115mu}{{\theta\left( d_{s} \right)}:={2 \cdot}}}\end{matrix}{{atan}\left( \frac{0.5d_{s}}{H_{a}} \right)}$${D_{main\_ ref}\left( d_{spot} \right)}:={1.27 \cdot \frac{\lambda}{\theta_{3{dB}}\left( d_{spot} \right)}}$$\begin{matrix}{\eta:={60\%}} & {\mspace{169mu}{{A_{main\_ ref}\left( d_{spot} \right)}:=\frac{\pi \cdot \left( {D_{main\_ ref}\left( d_{spot} \right)} \right)^{2}}{4}}}\end{matrix}$ A_(e)(d_(spot)) := A_(main_ref)(d_(spot)) ⋅ η$\begin{matrix}{{G_{a}\left( d_{spot} \right)}:={\frac{4\;\pi}{\lambda^{2}} \cdot {A_{e}\left( d_{spot} \right)}}} & {\mspace{56mu}{{G_{a\_ dB}\left( d_{spot} \right)}:={10\mspace{20mu}{\log\left( {G_{a}\left( d_{spot} \right)} \right)}}}}\end{matrix}$ G_(a_dB)(5 ⋅ mi) = 82.98 G_(a_dB)(10 ⋅ mi) = 76.959G_(a_dB)(15 ⋅ mi) = 73.437

For 25 miles spot size:

$\begin{matrix}{{G_{a\_ dB}\left( {25 \cdot {mi}} \right)} = 69} & {{mil\_ rad}:=\frac{rad}{1000}} \\{{A_{main\_ ref}\left( {25 \cdot {mi}} \right)} = {948.265\mspace{14mu} m^{2}}} & {{\mu\_ rad}:={10^{- 6} \cdot {rad}}} \\{{D_{main\_ ref}\left( {25 \cdot {mi}} \right)} = {948.265\mspace{14mu} m^{2}}} & \; \\{{\theta_{3{dB}}\left( {25 \cdot {mi}} \right)} = {0.063\mspace{14mu}\deg}} & {{\theta_{3{dB}}\left( {25 \cdot {mi}} \right)} = {1.096\mspace{14mu}{mil\_ rad}}}\end{matrix}$Assuming peak power needed for detection

$\begin{matrix}{P_{peak}:={5000\mspace{14mu}{watt}}} & {\mspace{11mu}{N_{element}:={{floor}\left( \frac{P_{peak}}{P_{element}} \right)}}} & {\mspace{14mu}{N_{element} = 625}} \\{P_{element}:={8 \cdot {watt}}} & \; & \;\end{matrix}$

Beam Pointing Resolution

The beam pointing resolution determines the amount of movement on theground required to keep a target area illuminated by the receiver (suchas an attack aircraft). For example, for the 25 mile diameter targetarea the phased array may move the illuminated area in steps of 5 or 10miles.

$\begin{matrix}{{{(s)}:={{floor}\left( \frac{D_{main\_ ref}\left( {25\mspace{14mu}\text{·~~}{mi}} \right)}{s} \right)}}\mspace{34mu}} & {{Number}\mspace{14mu}{of}\mspace{14mu}{Elements}} \\{{\theta_{BB}:={\frac{50.8}{\left( \frac{D_{main\_ ref}\left( {25 \cdot {mi}} \right)}{\lambda} \right)} \cdot \deg}}\mspace{65mu}} & {{Broadside}\mspace{14mu}{Beamwidth}}\end{matrix}$ $\begin{matrix}{\psi:={1 \cdot \deg}} & {\mspace{40mu}{S_{ground}:={\psi \cdot H_{alt}}}} & {\mspace{40mu} S_{ground}}\end{matrix} = {397.935\mspace{20mu} m}$

Translation on the Ground

The remaining calculations are based on basic design methodology of thephased array antenna based on the desired amount of beam translation onthe ground. By defining the amount of beam step on the ground(translation), there is the ability to proceed with the design of aCassegrain system. The calculations assume an F1 system wherein thediameter of the main reflector (reflector film) is the same as the focallength. Based on this the number of phase elements on the sub reflectoris calculated gives a particular phase shifter, i.e. the number of bitsin the phase shift.

$\begin{matrix}{S_{g\_ desired}:={20 \cdot m}} & {:=\frac{\lambda}{2}}\end{matrix}{element}\mspace{14mu}{spacing}$ $\begin{matrix}{\psi_{\delta}:=\frac{S_{g\mspace{14mu}{desired}}}{H_{alt}}} & \psi_{\delta}\end{matrix} = {877.193\;{\mu\_ rad}}$

For a Cassegrain system the angle scanned by the subreflector is halfthe total angle scanned by the main reflector.Ψ_(sr):=1·degAngle off boresight for subreflector

For an F1 optical system the diameter at the main reflector is the sameas the focal length.

$\begin{matrix}{D_{{reflector}\;}:={0.8{D_{main\_ ref}\left( {25 \cdot {mi}} \right)}}} & {D_{reflector} = {27.798\mspace{14mu} m}} \\{{const}:1.5} & \; \\{{focal}:{{const} \cdot D_{reflector}}} & {{focal} = {41.697\mspace{14mu} m}} \\{:=\frac{D_{reflector}}{focal}} & {\theta_{BB} = {38.197\mspace{14mu}\deg}}\end{matrix}$ $\begin{matrix}{\theta_{B}:=\frac{\theta_{BB}}{\cos\left( \psi_{ST} \right)}} & {\theta_{B} = {38.203\mspace{14mu}\deg}} & {{Scanned}\mspace{14mu}{beamwidth}}\end{matrix}$ $\begin{matrix}{p:=7} & {\mspace{211mu}{\#\mspace{14mu}{of}\mspace{14mu}{bits}\mspace{14mu}{of}\mspace{14mu}{phase}\mspace{14mu}{shifter}}}\end{matrix}$ $\begin{matrix}{{{\delta\;{\theta(s)}}:=\frac{9.0_{B}}{{N(s)} \cdot 2^{P}}}\mspace{79mu}} & {\mspace{20mu}{{beam}\mspace{14mu}{pointing}\mspace{14mu}{resolution}}}\end{matrix}$ $\begin{matrix}{N_{elements}:={{floor}\left( \frac{9 \cdot \theta_{B}}{\frac{\psi_{s}}{2} \cdot 2^{P}} \right)}} & {N_{elements} = 106}\end{matrix}$ $N_{elements}\begin{matrix}{{\cdot \frac{\lambda}{2}} = {1.59\mspace{14mu} m}} & {{Size}\mspace{14mu}{of}\mspace{14mu}{subreflector}}\end{matrix}$

It is to be understood that the foregoing detailed description isexemplary and explanatory only and is not to be viewed as beingrestrictive of embodiments of the invention, as claimed. The inventionis capable of other and different embodiments, and its several detailsare capable of modifications in various obvious respects, all withoutdeparting from the invention. Accordingly, the drawings and descriptionare to be regarded as illustrative in nature, and not as restrictive.Thus the scope of this invention should be determined by the appendedclaims, drawings and their legal equivalents.

1. A phased array balloon antenna comprising: at least one innermembrane coupled to an outer membrane; at least one phased array antennaconnected to each said inner membrane; said outer membrane having atleast one reflective film; said outer membrane being inflatable; eachsaid phased array antenna transmitting a radar energy towards each saidreflective film, said radar energy being reflected outwards from saidreflective film and illuminating at least one target area, wherein eachsaid target area is smaller than an area illuminated by said phasedarray antenna; and a shape of each said reflective film changeable byinflation of at least one compartment within said outer membrane,thereby adjusting each said target area illuminated by said phased arrayballoon antenna.
 2. The phased array balloon antenna of claim 1 whereinsaid balloon antenna is deployed on a satellite.
 3. The phased arrayballoon antenna of claim 1 wherein each said inner membrane is locatedon a side of said outer membrane opposite of each said reflective film.4. The phased array balloon antenna of claim 1 wherein each said innermembrane is inflatable.
 5. The phased array balloon antenna of claim 1wherein each said reflective film is suspended across a volumeencompassed by said outer membrane.
 6. The phased array balloon antennaof claim 1 wherein said shape of each said reflective film is changed byadjusting an inflation pressure differential between a small compartmentand a large compartment on either side of each said reflective filmwithin said outer membrane.
 7. A method for illuminating an area withradar energy comprising: providing a balloon antenna comprising at leastone inner membrane and an outer membrane, at least one phased arrayantenna connected to each said inner membrane, said outer membranehaving at least one reflective film; said outer membrane beinginflatable; transmitting a radar energy from each said phased arrayantenna towards each said reflective film, reflecting said radar energyoutwards from each said reflective film and illuminating at least onetarget area, wherein each said target area is smaller than an areailluminated by each said phased array antenna; and changing a shape ofeach said reflective film by changing the inflation of at least onecompartment within said outer membrane, thereby adjusting each saidtarget area illuminated by said phased array balloon antenna.
 8. Themethod of claim 7 wherein said balloon antenna is deployed on asatellite.
 9. The method of claim 7 wherein each said inner membrane islocated on a side of said outer membrane opposite of each saidreflective film.
 10. The method of claim 7 wherein each said innermembrane is inflatable.
 11. The method of claim 7 wherein each saidreflective film is suspended across a volume encompassed by said outermembrane.
 12. The method of claim 7 wherein said changing a shape ofeach said reflective film is achieved by adjusting an inflation pressuredifferential between a small compartment and a large compartment oneither side of each said reflective film within said outer membrane.